1. Field of the Invention:
The invention relates in general to electrical switches, and more specifically, to load-break, under-oil type switches suitable for immersion in the liquid dielectric of a distribution transformer.
2. Description of the Prior Art:
U.S. Pat. Nos. 3,461,259 and 3,590,183, which are assigned to the same assignee as the present application, disclose load-break switches suitable for use in loop type primary distribution systems. Two such switches are disposed in each distribution transformer of the loop, with the switches being immersed in the liquid dielectric of the transformer. The primary winding of each transformer is selectively connectable to either or both sides of the distribution loop by the two switches. The quick-make, quick-break action of the switch enables the switch to be closed on a fault, as well as to be opened while the transformer is carrying load current.
While switches constructed according to the teachings of these patents have performed very well, I have made certain improvements to the basic switch construction related to contact alignment, and the ability to increase contact pressure during momentary high current surges. These improvements are disclosed in detail in my U.S. Pat. No. 4,412,116, issued Oct. 25, 1983, which is assigned to the same assignee as the present application.
I have also observed that with the over-center type operating mechanism utilized by the switch of the hereinbefore-mentioned patents, a spring holds the switch open, and the spring also holds the switch closed. With the quick make-quick break action essential in order for the switch to pass load interruption and fault close-in tests, large mechanical forces must be absorbed on close-in, resulting in some contact rebound due to the characteristics of the spring. Also, with an over-center operating mechanism, it is possible to operate the mechanism to an intermediate balance point, i.e., precisely on center, without causing movement of the movable contacts. Thus, the position of the operating handle and the associated switch position indicator would indicate that the switch is in a different position than it actually is. Finally, the over-center operating mechanism of the switch disclosed in the hereinbefore-mentioned patents is only suitable for use with a two-position switch. Some applications for switching the primary of a distribution transformer in a loop feed require an additional switch position for grounding the distribution cable. Thus, it would be desirable to provide a new and improved quick-make, quickbreak switch having an operating mechanism which may be used with at least two and three position switches. It would also be desirable for such an operating mechanism to operate with the requisite snap action, without contact rebound. Finally, it would be desirable for the contact position indicator to always correctly indicate the actual switch position.